System and Method for Determining a Pilot Signal

ABSTRACT

A method of configuring a pilot signal includes defining a first pilot signal arrangement and defining a second pilot signal arrangement. Also, the method includes determining, by a communications controller, a first determined pilot signal arrangement in accordance with the first defined pilot signal arrangement, the second defined pilot signal arrangement, and a set of characteristics and transmitting, by the communications controller, the pilot signal having the first determined pilot signal arrangement.

This application is a continuation of U.S. Non-Provisional applicationSer. No. 13/798,297 filed on Mar. 13, 2013 and entitled “System andMethod for Determining a Pilot Signal,” which is hereby incorporated byreference herein as if reproduced in its entirety.

TECHNICAL FIELD

The present invention relates generally to telecommunications, and inparticular, to a system and a method for determining a pilot signal.

BACKGROUND

A long-term evolution (LTE) system, marketed as 4G LTE, is a standardfor high speed wireless communications for mobile phones and dataterminals. LTE is based on Global System for Mobile Communications (GSM)Enhanced Data Rates for GSM Evolution (EDGE) and Universal MobilTelecommunications System (UMTS) High Speed Network Access (HSPA)network technologies. An LTE system has an increased capacity and speed.The LTE system uses a different radio interface with core networkimprovements, such as using new digital signal processing (DSP)techniques. LTE is developed by the 3^(rd) Generation PartnershipProject (3GPP). The LTE system has a high spectral efficiency, very lowlatency, supports variable bandwidth, and has a simple architecture.

In telecommunications systems, such as an LTE system, a pilot signal ordemodulation reference signal (DMRS) is a signal, usually having asingle frequency, transmitted over a communications system forsupervision, control, equalization, continuity, synchronization, orreference purposes.

SUMMARY OF THE INVENTION

In accordance with an embodiment, a method of configuring a pilot signalincludes defining a first pilot signal arrangement and defining a secondpilot signal arrangement. Also, the method includes determining, by acommunications controller, a first determined pilot signal arrangementin accordance with the first defined pilot signal arrangement, thesecond defined pilot signal arrangement, and a set of characteristicsand transmitting, by the communications controller, the pilot signalhaving the first determined pilot signal arrangement.

In accordance with another embodiment of the present invention, a methodof configuring a pilot signal includes defining a first pilot signalarrangement and defining a second pilot signal arrangement. Also, themethod includes determining, by a user equipment, a first determinedpilot signal arrangement in accordance with the first defined pilotsignal arrangement, the second defined pilot signal arrangement, and aset of characteristics and communicating, by the user equipment, thepilot signal having the first determined pilot signal arrangement.

In a further embodiment of the present invention, a method ofconfiguring a pilot signal includes receiving, by a node, a set ofcharacteristics and determining a frequency domain density of the pilotsignal in accordance with the set of characteristics, a first pilotsignal arrangement, and a second pilot signal arrangement. Also, themethod includes communicating, by the node, the pilot signal at thedetermined frequency domain density.

In an additional embodiment, a method of configuring a pilot signalincludes receiving, by a node, a set of characteristics and determininga time domain density of the pilot signal in accordance with the set ofcharacteristics a first pilot signal arrangement, and a second pilotsignal arrangement. Also, the method includes communicating, by thenode, the pilot signal at the determined time domain density.

The foregoing has outlined rather broadly the features of an embodimentof the present invention in order that the detailed description of theinvention that follows may be better understood. Additional features andadvantages of embodiments of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiments disclosed may be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1 illustrates three frequency domain densities of pilot signals;

FIG. 2 illustrates three time domain densities of pilot signals;

FIG. 3 illustrates a graph of spectral efficiency versus SNR;

FIG. 4 illustrates an embodiment system for determining a pilot signalarrangement;

FIG. 5 illustrates an embodiment method of determining a pilot signalarrangement;

FIG. 6 illustrates another embodiment method of determining a pilotsignal arrangement;

FIG. 7 illustrates an additional method of determining a pilot signalarrangement

FIG. 8 illustrates a flowchart for an embodiment method of determining apilot signal arrangement;

FIG. 9 illustrates a table for an embodiment method of determining apilot signal arrangement;

FIG. 10 illustrates time interpolation for TTIs; and

FIG. 11 illustrates a schematic diagram of an embodiment of ageneral-purpose computer system.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

It should be understood at the outset that although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

In a legacy LTE system, there is a fixed pilot density that consumesabout 7% or 14% of download resources, which is a significant amount ofresources. A variable pilot density generally should consume fewerdownload resources.

FIG. 1 illustrates three examples of pilot densities in the frequencydomain, where the x axis represents time and the y axis representsfrequency. The dark blocks represent the symbols that the pilot is beingtransmitted on. High frequency density 100 has a density of 3/12orthogonal frequency division multiplexing (OFDM) tones, medium density102 has a density of 2/12 OFDM tones, and low density 104 has a densityof 1/12 OFDM tones. Similarly, FIG. 2 illustrates three examples ofpilot densities in the time domain. High time density 110 has a densityof 4/14 OFDM symbols, medium time density 112 has a density of 2/14 OFDMsymbols, and low density 114 has a density of 1/14 OFDM symbols.

In an example, the pilot density in the frequency domain and/or in thetime domain is independently determined by both a communicationcontroller and a user equipment using the same criteria, so theydetermine the same pilot density. The pilot density of the communicationcontroller and user equipment may be determined without communicatingthe pilot density between the communication controller and the userequipment. Characteristics used to determine the pilot density may becommunicated between the communication controller and the userequipment.

A communication controller may be a device configured to regulate thecommunications occurring in a communications system. Examples ofcommunications controllers include an evolved nodes B (eNB), a switchcoupled to and controlling the eNBs, a base station, a transmit point, aremote radio head, a communications controller, a controller, and thelike. Also, examples of user equipment include a mobile station, asubscriber, a user, a terminal, a phone, and the like.

Adaptive pilot density generally will reduce the pilot density. Byreducing the pilot density in the time and frequency domains, moreresource elements (REs) may be used for data transmission. Thus, betterthroughput and spectral efficiency may be achieved. FIG. 3 illustrates agraph of spectral efficiency versus signal to noise ratio (SNR). In theexample depicted in FIG. 3, there are 5 resource blocks (RBs), 4communication controller antennas, 4 user equipment antennas, closedloop spatial multiplexing (CLSM), one layer, and pedestrian type B (PB)channel. Curve 190 illustrates the spectral efficiency for a system witha fixed pilot density, while curve 192 illustrates spectral efficiencyfor a system with adaptive pilot density. Table I below illustrates thepilot density gains from an adaptive pilot density compared to a fixedpilot density in the time and frequency domain. The baseline is a hightime density and a high frequency density. For a high pilot density inthe frequency domain, there is a 0.0% gain with a high pilot density inthe time domain, a 12.1% gain for a medium pilot density in the timedomain, and an 18.1% gain for a low pilot density in the time domain.Also, for a medium pilot density in the frequency domain, there is a3.4% gain for a high pilot density in the time domain, a 13.8% gain fora medium pilot density in the time domain, and a 19.1% gain for a lowpilot density in the time domain. Further, for a low pilot density inthe frequency domain, there is a 6.9% gain for a high pilot density inthe time domain, a 15.5% gain for a medium pilot density in the timedomain, and a 19.8% gain for a low pilot density in the time domaindensity.

TABLE 1 Pilot density Gains Time domain density High Medium LowFrequency High 0.0% 12.1% 18.1% domain Medium 3.4% 13.8% 19.0% densityLow 6.9% 15.5% 19.8%

Pilot density may be adaptive only in the frequency domain, only in thetime domain, or in both time and frequency domains. Pilot density may becommunicated between a communication controller and a user equipment ina number of ways. In one example, the communication controllerdetermines the pilot density and transmits the pilot density to the userequipment at every starting time of a transmission. In another example,the user equipment determines the pilot density and transmits the pilotdensity to the communication controller at every starting time of atransmission. Alternatively, the communication controller and the userequipment share the same decision procedure based on a set ofpre-defined pilot configurations. The communication controller and theuser equipment may independently execute the procedure, where signalingdoes not happen at every starting time of a transmission. In oneembodiment, the pilot density is determined based on a tree structure.In another embodiment, the pilot density may be determined using alook-up table (LUT). The network and the communication controllers maydetermine the pilot density by sharing a common decision procedurewithout frequent signaling or feedback overhead. In an example, thecommunication controller and the user equipment both independentlydetermine the pilot density without signaling the pilot density. Eitherthe communication controller or the user equipment may initially set theprocedure used to determine the pilot density, and transmit thisprocedure initially. After the communication controller and the userequipment have this procedure, they can both independently determine thepilot density based on the same characteristics and have the sameresulting pilot density. The communication controller and the userequipment may transmit the characteristics used in determining the pilotdensity.

FIG. 4 illustrates system 120 for adaptively determining a pilotdensity. System 120 includes user equipment 122 and communicationcontroller 124. User equipment 122 transmits messages to communicationcontroller 124, and communication controller 124 sends messages to userequipment 122 to exchange characteristics that may be used to determinethe pilot density. Factors used by system 120 in determining the pilotdensity include network factors, user equipment factors, andenvironmental factors. Network factors include one or more of thetransmission type, the user equipment's assigned resource block (RB)size, the modulation and coding scheme (MCS) level, and the userequipment's assigned TTI length. User equipment factors include the userequipment's mobility and the user equipment's channel estimationcapability. Factors that affect the user equipment's channel estimationcapability include the number of user equipment antennas and the channelestimation algorithm used by the user equipment. Environmental factorsinclude the channel delay spread.

FIG. 5 illustrates flowchart 200 showing a method of adaptivelydetermining a pilot signal arrangement by a communications controller.Initially, in step 201, a pilot signal arrangement pool and proceduresare defined by a communications controller. For example, the pilotsignal arrangement pool might include a high pilot density arrangement,a medium pilot density arrangement, and a low pilot density arrangementin the time domain, in the frequency domain, or in both the time domainand the frequency domain. The pilot signal arrangement procedure may bea look-up table (LUT) or a tree decision structure. The procedures use aset of characteristics to determine pilot densities. This set ofcharacteristics can be divided into a first subset of characteristics,whose values are determined by the user equipment, and a second subsetof characteristics, whose values are determined by the communicationcontroller. Then, the communications controller determines the values ofthe second subset of characteristics in step 202. In step 204, the userequipment may determine and feed back the values of the first subset ofcharacteristics to the communication controller. Next, in step 182, thecommunication controller determines the pilot signal arrangement fromthe pilot signal arrangement pool using the pilot signal arrangementprocedure and the characteristics. After determining the pilot signalarrangement, the communications controller signals the pilot signalarrangement to the user equipment in step 206. Then, in step 208, thecommunications controller transmits the pilot signal to the userequipment. Finally, in step 210, the user equipment detects a signalbased on the pilot signal received from the user equipment.

FIG. 6 illustrates flowchart 220 for a method of adaptively determininga pilot signal arrangement by a user equipment. Initially, in step 221,the user equipment defines the pilot signal arrangement pool andprocedure. Then, in step 222, the user equipment determines the valuesof a first subset of characteristics to be used by the pilot signalprocedure. The communications controller may determine and signal thevalues of a second subset of characteristics to the user equipment.Next, in step 178, the user equipment determines the pilot signalarrangement from the pilot signal arrangement pool using the pilotsignal arrangement procedure and the characteristics. After determiningthe pilot signal arrangement, the user equipment feeds back the pilotsignal arrangement to the communications controller in step 226. Then,in step 208, the communications controller transmits a pilot signal tothe user equipment. Finally, in step 210, the user equipment detects asignal based on the received pilot signal.

FIG. 7 illustrates flowchart 170 depicting a method of adaptivelydetermining a pilot signal arrangement. Initially, in step 172, pilotsignal arrangement pool and procedures are defined. This may be done bya communication controller, a user equipment, or both. The pilot signalprocedures define how a pilot signal arrangement is selected from a setof pilot signal arrangement pool. For example, the pilot signalarrangement pool may include a high density arrangement, a mediumdensity arrangement, and a low density arrangement in both the frequencydomain and the time domain. Then, in step 174, the node that defined thepilot signal arrangements and procedures shares the pilot signalarrangement pool and procedures with other nodes. For example, if a userequipment defined the pilot signal arrangement pool and procedures, theuser equipment transmits them to the communication controller. However,if a communication controller developed the pilot signal arrangementpool and procedures, the communication controller transmits them to theuser equipment.

Then, in step 176, the communication controller determines and signalscharacteristics to the user equipment, and in step 178, the userequipment determines the pilot signal arrangement based on the pilotsignal arrangement pool, the pilot signal arrangement procedure, andcharacteristics. Also, in step 180, the user equipment determines andfeeds back characteristics to the communication controller. Next, instep 182, the communication controller determines the pilot arrangementbased on the characteristics, and the pilot signal arrangement pool andprocedures. Hence, the communication controller and user equipment maydetermine the same pilot signal arrangement without directlycommunicating the pilot signal arrangement at every starting time oftransmission, because they use the same pilot signal arrangementprocedures, pilot signal arrangement pool, and characteristics withoutdirectly communicating the pilot signal arrangement. In one example,only one of step 176 and step 180 are performed. Alternatively, bothstep 176 and step 180 are performed. Then, in step 184, thecommunication controller transmits the pilot signal arrangement to theuser equipment. Finally, in step 186, the user equipment detects asignal based on the received pilot signal.

In one embodiment, the frequency domain pilot density is determinedbased on the channel delay spread, the transmission type, thecommunication controller capability, the RB size, and the MCS level. Thefrequency domain pilot density may be determined using a tree structure.Alternatively, the frequency domain pilot density may be determinedusing a LUT. FIG. 8 illustrates a tree structure that can be used todetermine the pilot density in the frequency domain. Alternatively,other decision trees or LUTs may be used. Initially, in step 130, thechannel delay spread is determined. When the channel delay spread islong, for example with a root mean square (RMS) of greater than 7 μs,high pilot density 144 is used. However, when the channel delay spreadis short, for example with an RMS value of less than or equal to 7 μs,the transmission type is further evaluated in step 132. In step 132,when the transmission type is joint transmission (JT), high pilotdensity 144 is used, and when the transmission type is not JTtransmission, the communication controller capability is furtherevaluated in step 134. Then, in step 134, when the communicationcontroller capability is regular, RB size is further evaluated in step136. If the RB size is determined to be small in step 136, for exampleif the RB size is less than or equal to five RBs, high pilot density 144is used. However, if the RB size is determined to be large in step 136,for example if the RB size is greater than five RBs, medium pilotdensity 146 is used. When the communication controller capabilitydetermined in step 134 is strong, the RB size is determined in step 138.When the RB size is determined to be small in step 138, the MCS level isevaluated in step 140, and when the RB size is determined to be large instep 138, the MCS level is evaluate in step 142. In step 140, if the MCSlevel in 64-QAM or 16-QAM, medium pilot density 146 is used, while ifthe MCS level is QPSK, low pilot density 148 is used. However, in step142, if the MCS level is 64-QAM, medium pilot density 146 is used, whileif the MCS level is QPSK or 16-QAM, low pilot density 148 is used.

Alternatively, the time domain pilot density may be determined using aLUT. FIG. 9 illustrates a table 150 illustrating a LUT used to determinethe time domain pilot density. However, other LUTs or a tree structuresmay be used to determine the time domain pilot density. When the userequipment's mobility is low and the TTI length is long, a low pilotdensity is used. For example, a low mobility user equipment may be fixedor nomadic, moving at a speed of less than 30 km/hour. A high mobilityuser equipment may be moving at a speed of greater than or equal to 30km/hour. In an example, a long TTI length includes 56 OFDM symbols, amedium TTI length includes 28 OFDM symbols, and a short TTI lengthincludes 14 OFDM symbols. When the user equipment mobility is low andthe TTI length is medium or short, a medium pilot density is used. Also,when the user equipment mobility is high and the TTI length is long, amedium pilot density is used. However, when the user equipment mobilityis high and the TTI length is medium or short, a high pilot density isused.

In an example, when a user equipment enters a network, it transmits itschannel estimation capability to the network. At every datatransmission, after the network determines the MCS level and RB size,the communication controller may determine the pilot density. After theuser equipment knows the MCS level and RB size, for example from thephysical downlink control channel (PDCCH), the user equipment maydetermine the pilot density. In addition, explicit signalinginformation, such as a transmission mode, may be used to help thedensity decision. The user equipment may demodulate data based on thepilot density.

A user equipment may be aware that it will potentially be served byjoint transmission. To enable joint transmission, a user equipment'sfeedback has a special mode, for example based on user equipmentfeedback and an inter-communications controller feedback. Once a jointtransmission mode is configured by the network, the communicationscontroller knows that it will potentially be served by jointtransmission.

A channel delay spread is mainly determined by the environment aroundthe communication controller. User equipments in the vicinity of thecommunication controller have similar channel delay spreads. Acommunication controller may estimate the common channel delay spread byaveraging the channel delay spread from all neighboring user equipment.For example, a communication controller may estimate the channel delayspread based on the cyclic prefix (CP) of the user equipment's uplinksignal. Based on the uplink signals from all user equipments received byone communication controller, the communication controller may estimatethe channel delay spread and determine its category. A communicationcontroller can accumulate the upload signals of many user equipments toaverage channel randomness and obtain a good channel delay spreadestimation. After estimation, the channel delay spread can be stored inthe communication controller. The channel delay spread may be updated asneeded.

A user equipment's channel estimation capability may be categorized asregular or strong. A user equipment may determine its own capabilitybased on a standard method or metric, for example based on the signal tonoise ratio (SNR) loss due to channel estimation.

The user equipment's mobility and TTI length affect the pilot density inthe time domain. When a user equipment has a low mobility, thecommunication controller can reduce the pilot density in the timedomain. When a user equipment has a low mobility, its channel variesslowly in time, and the pilot density in the time domain can be reduced.With a long TTI length, good channel estimation is still achieved bytime interpolation even with a lower pilot density. However, with ashort TTI length, the same reduction can be made, because a userequipment can store the received signals of several past TTIs. Ifseveral consecutive TTIs are given to one user equipment, using pilotsof the past TTIs, the user equipment may perform interpolation to assistin channel estimation. FIG. 10 illustrates diagram 160 showing storedpilot signals from past TTIs that may be used for time interpolation.

FIG. 11 illustrates a block diagram of processing system 270 that may beused for implementing the devices and methods disclosed herein. Specificdevices may utilize all of the components shown, or only a subset of thecomponents, and levels of integration may vary from device to device.Furthermore, a device may contain multiple instances of a component,such as multiple processing units, processors, memories, communicationcontrollers, user equipments, etc. The processing system may comprise aprocessing unit equipped with one or more input devices, such as amicrophone, mouse, touchscreen, keypad, keyboard, and the like. Also,processing system 270 may be equipped with one or more output devices,such as a speaker, a printer, a display, and the like. The processingunit may include central processing unit (CPU) 274, memory 276, massstorage device 278, video adapter 280, and I/O interface 288 connectedto a bus.

The bus may be one or more of any type of several bus architecturesincluding a memory bus or memory controller, a peripheral bus, videobus, or the like. CPU 274 may comprise any type of electronic dataprocessor. Memory 276 may comprise any type of system memory such asstatic random access memory (SRAM), dynamic random access memory (DRAM),synchronous DRAM (SDRAM), read-only memory (ROM), a combination thereof,or the like. In an embodiment, the memory may include ROM for use atboot-up, and DRAM for program and data storage for use while executingprograms.

Mass storage device 278 may comprise any type of storage deviceconfigured to store data, programs, and other information and to makethe data, programs, and other information accessible via the bus. Massstorage device 278 may comprise, for example, one or more of a solidstate drive, hard disk drive, a magnetic disk drive, an optical diskdrive, or the like.

Video adaptor 280 and I/O interface 288 provide interfaces to coupleexternal input and output devices to the processing unit. Asillustrated, examples of input and output devices include the displaycoupled to the video adapter and the mouse/keyboard/printer coupled tothe I/O interface. Other devices may be coupled to the processing unit,and additional or fewer interface cards may be utilized. For example, aserial interface card (not pictured) may be used to provide a serialinterface for a printer.

The processing unit also includes one or more network interface 284,which may comprise wired links, such as an Ethernet cable or the like,and/or wireless links to access nodes or different networks. Networkinterface 284 allows the processing unit to communicate with remoteunits via the networks. For example, the network interface may providewireless communication via one or more communicationcontrollers/transmit antennas and one or more user equipments/receiveantennas. In an embodiment, the processing unit is coupled to alocal-area network or a wide-area network for data processing andcommunications with remote devices, such as other processing units, theInternet, remote storage facilities, or the like.

Advantages of an embodiment include using adaptive pilot density toreduce overhead and exploit the diversity of communication controllers.In an embodiment, the network and communication controllers determinethe pilot density by sharing a common decision procedure withoutfrequent signaling or feedback overhead. An advantage of an embodimentincludes the reduction of signaling and/or feedback overhead.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A method of transmitting a pilot signal, themethod comprising: transmitting, by a transmitter, a signal indicating afirst pilot density arrangement of multiple pilot density arrangementsin a time domain, the multiple pilot density arrangements furtherincluding a second pilot density arrangement different from the firstpilot density arrangement, the first pilot density arrangementassociated with a first transmission time interval (TTI) length, and thesecond pilot density arrangement associated with a second TTI length. 2.The method of claim 1, wherein the first TTI length is equal to a firstnumber of symbols and the second TTI length is equal to a second numberof symbols.
 3. The method of claim 2, wherein the first TTI length isequal to 14 symbols and the first pilot density arrangement includes 4pilot symbols per TTI.
 4. The method of claim 1, further comprising:upon transmitting the signal, transmitting, by the transmitter, thepilot signal based on either the first pilot density arrangement or thesecond pilot density arrangement being configured for a datatransmission.
 5. The method of claim 4, wherein the pilot signal is ademodulation reference signal (DMRS).
 6. The method of claim 1, wherein:the multiple pilot density arrangements further include a third pilotdensity arrangement associated with a third TTI length that is equal toa third number of symbols; the first pilot density arrangement, thesecond pilot density arrangement, and the third pilot densityarrangement are different from each other; and the first TTI length, thesecond TTI length, and the third TTI length are different from eachother.
 7. The method of claim 1, wherein the first pilot densityarrangement is determined in accordance with a set of characteristics.8. The method of claim 7, wherein the set of characteristics include afirst subset of characteristics and a second subset of characteristics,the first subset of characteristics including a network or environmentalcharacteristic, and the second subset of characteristics including anenvironmental or user equipment characteristic.
 9. The method of claim8, wherein the environmental characteristic includes a channel delayspread characteristic.
 10. A method of receiving a pilot signal, themethod comprising: receiving, by a user equipment (UE), a signalindicating a first pilot density arrangement of multiple pilot densityarrangements in a time domain, the multiple pilot density arrangementsfurther including a second pilot density arrangement different from thefirst pilot density arrangement, the first pilot density arrangementassociated with a first transmission time interval (TTI) length, and thesecond pilot density arrangement associated with a second TTI length.11. The method of claim 10, wherein the first TTI length is equal to afirst number of symbols and the second TTI length is equal to a secondnumber of symbols.
 12. The method of claim 11, wherein the first TTIlength is equal to 14 symbols and the first pilot density arrangementincludes 4 pilot symbols per TTI.
 13. The method of claim 10, furthercomprising: after receiving the signal, receiving, by the UE, the pilotsignal based on either the first pilot density arrangement or the secondpilot density arrangement being configured for a data transmission. 14.The method of claim 13, wherein the pilot signal is a demodulationreference signal (DMRS).
 15. The method of claim 10, wherein: themultiple pilot density arrangements further include a third pilotdensity arrangement associated with a third TTI length that is equal toa third number of symbols; the first pilot density arrangement, thesecond pilot density arrangement, and the third pilot densityarrangement are different from each other; and the first TTI length, thesecond TTI length, and the third TTI length are different from eachother.
 16. The method of claim 10, wherein first pilot densityarrangement is determined in accordance with a set of characteristics.17. The method of claim 16, wherein the set of characteristics include afirst subset of characteristics and a second subset of characteristics,the first subset of characteristics including a network or environmentalcharacteristic, and the second subset of characteristics including anenvironmental or user equipment characteristic.
 18. The method of claim17, wherein the environmental characteristic includes a channel delayspread characteristic.
 19. A transmitter comprising: a memory storagecomprising instructions; and a processor in communication with thememory storage, wherein the processor executes the instructions to:transmit a signal indicating a first pilot density arrangement ofmultiple pilot density arrangements in a time domain, the multiple pilotdensity arrangements further including a second pilot densityarrangement different from the first pilot density arrangement, thefirst pilot density arrangement associated with a first transmissiontime interval (TTI) length, and the second pilot density arrangementassociated with a second TTI length.
 20. The transmitter of claim 19,wherein the processor executes the instructions to: upon transmittingthe signal, transmit a pilot signal based on either the first pilotdensity arrangement or the second pilot density arrangement configuredfor a data transmission, the pilot signal being a demodulation referencesignal (DMRS).
 21. A user equipment (UE) comprising: a memory storagecomprising instructions; and a processor in communication with thememory storage, wherein the processor executes the instructions to:receive a signal indicating a first pilot density arrangement ofmultiple pilot density arrangements in a time domain, the multiple pilotdensity arrangements further including a second pilot densityarrangement different from the first pilot density arrangement, thefirst pilot density arrangement associated with a first transmissiontime interval (TTI) length, and the second pilot density arrangementassociated with a second TTI length.
 22. The UE of claim 21, wherein thefirst TTI length is equal to 14 symbols and the first pilot densityarrangement includes 4 pilot symbols per TTI.